The present invention relates to a structure of a headstock of a polishing machine for use in polishing the surfaces of flat-plate substrates, such as semiconductor substrates, magnetic disks and glass substrates.
A structure of a headstock of a conventionally-used polishing machine is schematically shown in FIG. 15.
A polishing pad 7 is adhered to an upper surface of a turntable 6. The headstock 10 is positioned above the turntable 6 so as to face it. The headstock 10 is essentially formed of a spindle 4 and a holder plate 3. The holder plate 3 is connected to the lower end of the spindle 4. A thin disk-form workpiece 2 is attached to the lower surface of the plate 3 with a soft pad 5 interposed between them.
The surface of the workpiece 2 is ground by pressing the workpiece 2 against the polishing pad 7 while the turntable 6 and the headstock 10 are separately rotated, and a polishing medium or slurry is supplied from a nozzle 8 to the polishing pad 7 placed on the turntable 6.
If the surface of the polishing pad 7 (hereinafter, referred to as "pad surface") is completely flat, the resultant ground surface of the workpiece will be completely flat. However, in fact, the polishing pad is non-uniform in thickness by nature. For example, a commercially available polishing pad varies several tens of micrometers in thickness even in the same pad. To improve the flatness of the polishing pad, dressing is applied to the polishing pad prior to polishing. However, even if dressing is applied, about 10 .mu.m difference in thickness still remains. Furthermore, the polishing pad is deformed by frictional heat or the influence of pressure. As a result, the workpiece 2 is not in uniform contact with the pad surface. The resultant ground surface of the workpiece 2 is degraded in flatness.
To reduce the aforementioned non-uniform contact of the workpiece with the pad surface, the following structure has been proposed. In this structure, the holding plate is not directly fixed to the lower end of the spindle. It is fixed to the lower end of the spindle via a tilting mechanism, thereby enabling the headstock to trace an uneven pad surface. Such a structure (shown in FIG. 9A) is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication Nos. 61-25767, 61-25768, and 61-4662.
A flange 78 having a spherical seats at the lower side, is provided at a lower end of a spindle 4. A hemispherical body 79 moving along the spherical seat is placed under the flange 78. A holder plate 12 is attached to the lower side of the hemispherical body 79. To transmit rotation torque from the spindle 4 to the holder plate 12, they are connected to each other via an Oldham coupling (Jpn. Pat. Appln. KOKAI Publication No. 61-25767), bellows (Jpn. Pat. Appln. KOKAI Publication No. 61-25768), or three links (Jpn. Pat. Appln. KOKAI Publication No. 61-4662).
However, in the aforementioned headstock, pressure is transmitted from the spindle 4 to the holder plate 12 via the sliding face, which is between the spherical seat and the hemispherical body 79. This means that the tilting movement of the holder plate may be disturbed by friction force at the sliding face. In addition, the center of gravity of the tilting portion (consisting of holder plate 12 and a hemispherical body 79) is far away from the center of tilting in a headstock of this type. Consequently, if the holder plate 12 is rotated, a centrifugal force works on the holder plate 12 and accelerates its tilting, with the result that the pad surface cannot be traced desirably. The influence of the centrifugal force cannot be negligible when a highly flat ground surface is desired.